Treatment of tailings streams with one or more dosages of lime, and associated systems and methods

ABSTRACT

Methods and systems for treating oil sands tailings streams using multiple dosages of lime are disclosed herein. In some embodiments, the method comprises providing a tailings stream including 3-40% solids by total weight, combining the tailings stream with a first dosage of lime to produce a first mixture having a pH of less than 12.0, and then combining the first mixture with a polymer to produce a second mixture. In some embodiments, the method can further include combining the second mixture with a second dosage of lime to produce a third mixture having a pH greater than 12.0, and dewatering the third mixture in a centrifuge unit and/or a pressure filtration unit to produce a product stream having 55% or more solids by weight.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional Application62/583,327, filed on Nov. 8, 2017 and incorporated herein by referencein its entirety.

TECHNICAL FIELD

This application relates to systems and methods for promoting dewateringof tailings streams using lime. In some embodiments, tailings streamsfrom oil sands or mining operations are mixed with a polymer and one ormore dosages of lime additive to promote dewatering of the tailingsstreams.

BACKGROUND

The extraction of bitumen from oil sands has been traditionallyperformed using the Clark Hot Water Extraction (CHWE) process orvariants thereof. A tailings slurry, defined as whole tailings, isproduced as a byproduct of the CHWE process, and can include water,sand, clay, and residual bitumen particles that are suspended in theextraction water. Coarse sand particles (e.g., >44 μm) can be easilyremoved from whole-tailings, but removal of finer particles (fines) canbe more problematic. A portion of the remaining fines, water, andresidual bitumen form a slurry that is about 10-15% solids by mass,which after a number of years can settle to be about 20-40% solids bymass. This slurry is referred to as fluid fine tailings (FFT) and/ormature fine tailings (MFT), and can remain for decades in a fluid statewithout further aggregation or settling. Slow consolidation, limitedsolids strength, and poor water quality of the FFT/MFT limits optionsfor reclamation and has resulted in the formation of large tailingsponds.

A number of different technologies have been tried to improve thereclamation of FFT/MFT. Some of these technologies includewhole-tailings treatment, non-segregating treatment (NST) production,composite tailings (CT) production, tailings reduction operations (TRO),atmospheric drying, or treatment with polymers. Furthermore, some ofthese technologies include treating the FFT/MFT using gypsum. Thesemethods, however, have worked with only limited success and manytechnologies yield treated tailings that require additional treatmentsbefore reclamation is possible. For example, when using gypsum to treatFFT and/or MFT, the resulting release water contains high concentrationsof soluble calcium, which can impair effectiveness of the subsequentextraction process. There currently exists over a billion cubic metersof FFT/MFT in tailings ponds. As such, there is a need for an improvedmethod and process to treat oil sands tailings to provide an effectivereclamation option.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a tailings dewatering system,configured in accordance with embodiments of the present technology.

FIG. 2A is a schematic flow diagram of a tailings dewatering system,configured in accordance with embodiments of the present technology.

FIG. 2B is a schematic flow diagram of an extraction process of thetailings dewatering system, configured in accordance with an embodimentof the present technology.

FIG. 3 is a block diagram of a method of dewatering a tailings stream,configured in accordance with an embodiment of the present technology.

FIG. 4 depicts a flow chart 400 for dewatering a tailings stream,configured in accordance with an embodiment of the present technology.

FIGS. 5A-5C are images of experimental results related to treatment oftailings stream samples with and without lime over a period of time,configured in accordance with embodiments of the present technology.

FIG. 6 is an image of experimental results related to treatment oftailings stream samples using various concentrations of lime, inaccordance with embodiments of the present technology.

DETAILED DESCRIPTION

A method and system of dewatering tailings streams using a single ormultiple dosages of a lime-based additive is described in detail hereinin accordance with embodiments of the present technology. Numerousspecific details are set forth in the following description and figuresto provide a thorough and enabling description of embodiments of thetechnology. One skilled in the relevant art, however, will recognizethat the technology can be practiced without one or more of the specificdetails. In other instances, well-known structures or operations are notshown or are not described in detail to avoid obscuring aspects of thetechnology. In general, alternatives and alternate embodiments describedherein are substantially similar to the previously describedembodiments, and common elements are identified by the same referencenumbers.

FIG. 1A is a schematic flow diagram of a tailings dewatering system 100(“system 100”). The system 100 includes a tailings holding reservoir 102(e.g., a pond, diked area, tank, etc.) including tailings 103 containedtherein, and a lime holding reservoir 104 (e.g., a tank) including alime slurry 105 (e.g., a lime additive) contained therein. The tailings103 can originate from oil sands operations, and generally include theremains of the oil sands after the extraction of bitumen therefrom. Forexample, tailings can include whole-tailings (WT), thin fluid tailings(TFT), fluid fine tailings (FFT), hydro-cyclone overflow or underflowand/or mature fine tailings (MFT) (referred to collectively as“tailings”). In addition, tailings can originate from the extraction ofminerals (e.g., copper, gold and/or uranium) from other miningoperations. The tailings 103 can come from the tailings holdingsreservoir 102 or directly from another process 101 (e.g., an extractionprocess) without being routed through the tailings holding reservoir102. The tailings 103 in the tailings holding reservoir 102 can beslightly alkaline, having a pH level of about 7.5-8.5, and the limeslurry 105 in the lime holding reservoir can be alkaline, having a pHlevel greater than or equal to about 12.0.

The tailings 103 is combined with a dosage of the lime slurry 105 in afirst vessel 106 (e.g., a mixer) to produce a lime-tailings mixture 107having a first composition and a pH equal to or less than about 12.0.For example, the pH of the lime-tailings mixture 107 may be equal to orless than about 11.9, 11.8, 11.7, 11.6 or 11.5. In addition to or inlieu of the foregoing, the amount of soluble calcium levels of thelime-tailings mixture 107 may be equal to or less than 30 mg/L, 25 mg/L,20 mg/L. The dosage of lime slurry 105 reacts with the tailings 103 toremove at least a portion of the bicarbonates present in the tailings.In some embodiments, the lime-tailings mixture 107 can then be diluted(e.g., with process water) to contain about 3% solids by weight.Dilution of the lime-tailings mixture 107 can help promote betterflocculation and speed the settling rate of flocculated solids.

As shown in the illustrated embodiment, after the tailings 103 and limeslurry 105 are combined, the lime-tailings mixture 107 is then combinedwith a flocculant 110. This can occur in-line via line 113 a and/or in asecond vessel 108 (e.g., a thickener or holding reservoir) via line 113b. The lime-tailings mixture 107 can separate (e.g., via settling) overa period of time to produce a first stream 109 substantially comprisingsolids, and a second stream 111 substantially comprising process water.In some embodiments, the addition of the flocculant 110 to thelime-tailings mixture 107 is based on (a) the first stream 109 beinggreater than 30% solids by weight and/or (b) the second stream 111 beingless than 3% solids by weight. For example, if the first stream 109 hasa solids content less than 30% solids by weight, the amount offlocculant 110 added to the lime-tailings mixture 107 may be increased.Additionally, if the second stream 111 has a solids content greater than3% solids by weight, the amount of flocculant 110 added to thelime-tailings mixture 107 may be increased.

The second stream 111 comprises water including solids levels at lessthan 3% and containing sodium hydroxide particles that have been formedas a byproduct of reacting the lime slurry 105 with bicarbonates fromthe tailings 103. As described in more detail below with respect to FIG.2B, the second stream 111 can be directed toward and used to promotebitumen extraction. The first stream 109, corresponding to a mixture(e.g., a second mixture) having a second composition, is removed from abottom portion of the vessel 108 and can be routed to further downstreamprocessing or a disposal area (e.g., a pit or diked disposal area). Insome embodiments, downstream processing can include drying (e.g.,Tailings Reduction Operations (TRO), Atmospheric Fines Drying (AFD) orrim ditch), or routing to a disposal area (e.g., a diked disposal area)or reclamation area. The reclamation area may be, for example, watercapped using Permanent Aquatic Storage Structure (PASS) technology.

The tailings 103 can include water, sand, clay, and residual bitumenparticles that are suspended in the extraction water. The tailings 103can be obtained from tailings ponds or steady-state extraction processesfrom oil sands or mining operations. The tailings 103 may be stored in atailings pond and include a settled solids content of about 10-45% byweight (e.g., wet weight). More specifically, the tailings can include amineral solids content from about 5-40%, a bitumen content from about0-3%, a clay content from about 40-100%, and a pH from about 7.5-9.0. Insome embodiments, the tailings 103 may undergo upstream processing(e.g., prior to being held in the holding reservoir 102), such ascyclone separation, screen filtering, thickening and/or dilutionprocesses. The tailings 103 entering the mixer 106, after potentiallybeing combined with recycled water 122, can be diluted to be as low as3% solids by weight. In some embodiments, the solids content ispreferably above 10% by weight.

The lime slurry 105 in the lime slurry holding reservoir 104 includes aliquid (e.g., water) and a lime additive that can be less than 15% byweight of the lime slurry, less than about 10% of the lime slurry, orless than about 5% of the lime slurry. The lime slurry additive 105stored in the lime holding reservoir 104 can include inorganic materialsthat provide divalent (e.g., calcium) cations. As such, the lime slurry105 can comprise a lime product including hydrated lime (e.g., calciumhydroxide (Ca(OH)₂)), slaked quicklime (e.g., calcium oxide (CaO)),and/or enhanced hydrated lime. The enhanced hydrated lime can includeparticles with an average Brunauer-Emmett-Teller (BET) surface areaexceeding 30 m²/g. Other specifications and characteristics of enhancedhydrated particles are described in U.S. patent application Ser. No.15/922,179, filed Mar. 15, 2018, the disclosure of which is incorporatedherein by reference in its entirety. In some embodiments, the limeslurry can include dolomitic lime (e.g., lime including at least 25%magnesium oxide on a non-volatile basis), other lime-containingmaterials, or a combination of quicklime, limestone, hydrated lime,enhanced hydrated lime, dolomitic lime, and/or other lime-containingmaterials. In the lime manufacturing process, limestone (e.g., calciumcarbonate (CaCO₃)), is crushed to ¼″ to 2″ particles used as kiln feed.The kiln feed is then calcined, which converts the limestone particlesinto calcium oxide, which is sometimes referred to as quicklime.Introducing water to the quicklime leads to the formation of fineparticles of hydrated lime, which is often referred to using the genericterm “lime.”

The tailings 103 and the lime slurry additive 105 are combined in themixer 106 to produce the lime-tailings mixture 107 having the firstcomposition. The mixer 106 can include means to agitate thelime-tailings mixture 107, such as rotating blades. In some embodiments,the mixer 106 can include a static mixer, a dynamic mixer, or a T mixer.The residence time in the mixer 108 for particles of the lime-tailingsmixture 107 can be, for example, at least about five seconds, at leastabout 60 seconds, at least about five minutes, at least about 10minutes, or at least about 20 minutes. In general, the mixer 106 mixesthe tailings 103 and lime slurry 105 to ensure the lime-tailings mixture107 exiting the mixer 106 is well mixed and has a desired pH at orslightly below about 12.0, 11.8 or 11.5. A pH at or below 12.0, forexample, can aid in minimizing the bicarbonates present in the tailings103. Additionally, a pH at or below 12.0 generally does not providesoluble calcium cations prior to the polymer 110 being added in asubsequent step. This minimizes the concentration of soluble calcium inthe second stream 111. In some embodiments, the soluble calcium cationsin stream 111 comprise about 100 mg/L, 90 mg/L, 80 mg/L, 70 mg/L, 60mg/L, 50 mg/L, 40 mg/L, 30 mg/L, 20 mg/L, about 10 mg/L, or less. The pHof the lime-tailings mixture 107 at the outlet of the mixer 106 can bemeasured and used to control the pH of the lime-tailings mixture 107 by(a) increasing or decreasing the feed rate of the incoming lime slurry105, and/or (b) increasing or decreasing the residence time of thetailings 103 and lime slurry 105 in the mixer 106.

The lime-tailings mixture 107 is directed to the second vessel 108 whereit is combined with the flocculant 110. The flocculant can include oneor more anionic, nonionic, cationic, or amphoteric polymers, or acombination thereof. These polymers can be naturally occurring (e.g.,polysaccharides) or synthetic (e.g., polyacrylamides). In someembodiments, the flocculant 110 can be added as a part of a slurry,which may comprise about 0.4% by weight flocculant and process waterand/or makeup water. Typically, at least one component of the flocculant110 will be high molecular weight (e.g., up to about 50,000 kD). In someembodiments, the flocculant can promote thickening (e.g., increasing theconcentration of solids) of the lime-tailings mixture 107 and allowsolids from the lime-tailings mixture 107 to settle faster compared toif the lime-tailings mixture was treated only with a lime additive oronly with a flocculant. The vessel 108 and the residence time of thelime-tailings mixture 107 in the vessel 108 can also promote thickeningof the lime-tailings mixture 107 by aiding in the separation of thelime-tailings mixture 107 into the first stream 109 and the secondstream 111. Stated otherwise, the vessel 108 decreases the amount ofwater that has to be removed by, for example, the dewatering device 116to obtain a cake having acceptable geotechnical properties. As a result,removing the second stream 111 from the first stream 109 can decreasecycle time of the overall dewatering process.

The system 100 can further include a control system 130. As described inmore detail below with reference to FIG. 2A, the control system 130 canbe used to control operation associated with the system 100. Manyembodiments of the control system 130 and/or technology described belowmay take the form of computer-executable instructions, includingroutines executed by a programmable computer. The control system 130may, for example, also include a combination of supervisory control anddata acquisition (SCADA) systems, distributed control systems (DCS),programmable logic controllers (PLC), control devices, and processorsconfigured to process computer-executable instructions. Those skilled inthe relevant art will appreciate that the technology can be practiced oncomputer systems other than those described herein. The technology canbe embodied in a special-purpose computer or data processor that isspecifically programmed, configured or constructed to perform one ormore of the computer-executable instructions described below.Accordingly, the term “control system” as generally used herein refersto any data processor. Information handled by the control system 130 canbe presented at any suitable display medium, including a CRT display orLCD.

The technology can also be practiced in distributed environments, wheretasks or modules are performed by remote processing devices that arelinked through a communications network. In a distributed computingenvironment, program modules or subroutines may be located in local andremote memory storage devices. Aspects of the technology described belowmay be stored or distributed on computer-readable media, includingmagnetic or optically readable or removable computer disks, as well asdistributed electronically over networks. Data structures andtransmissions of data particular to aspects of the technology are alsoencompassed within the scope of particular embodiments of the disclosedtechnology.

In some embodiments, the second mixture 109 can undergo furthertreatment(s), including one or more additional dosages of lime and/orflocculants. FIG. 2A, which illustrates one such embodiment, includes athird vessel 112 (e.g., a second mixer) to which the second mixture 109is routed to. As shown in the illustrated embodiment, the second mixture109 is combined with a second dosage of lime slurry 115 in a thirdvessel 112, e.g., via line 125 a, to produce a mixture 117 having athird composition and a pH greater than about 12.0, 12.2, or 12.4. Thesecond dosage of lime slurry 115 can originate from the lime holdingtank 104, a separate lime holding tank, or other means. As shown in theillustrated embodiment, the mixture 117 is then moved to a dewateringdevice 116 to promote dewatering of the mixture 117. As explained infurther detail below, the dewatering device 116 can include acentrifuge, and/or a pressure, belt or vacuum filtration system thatseparates the mixture 117 into a first stream 118 substantiallycomprising solids (e.g., a “cake”) and a second stream 120 substantiallycomprising a centrate or a filtrate (e.g., release water). The firststream 118 may be combined with a lime slurry, e.g., via line 125 b. Insome embodiments, the mixture 117 can be placed on one or more pads inthin/thick lifts to consolidate and dry the solids content containedtherein.

As further shown in the illustrated embodiment, the second stream 120can be directed to a pond and/or be used as a recycled stream 122. Therecycled stream 122 can be combined with (a) the tailings reservoir 102,e.g., via line 122 a, (b) the tailings 103, e.g., via line 122 b, priorto being mixed with the first dosage of lime slurry 105, (c) the limeslurry reservoir 104, e.g., via line 122 c, (d) the lime slurry 105,e.g., via line 122 d, prior to being mixed with the tailings 103, (e)the lime slurry reservoir 115, e.g., via line 122 e, (f) the mixture117, e.g., via line 122 f. The recycled portion of the release water caninclude soluble calcium cations previously injected as part of the limeslurry, and thus can decrease the amount of the first dosage of limeslurry 105 needing to be injected to the mixer 106. The second stream118 can be collected and transported using a truck, belt, pump, and/orother conveying system(s) to an external site (e.g., a temporary storageor reclamation area).

As noted above, the second stream 111 can be directed toward and used topromote bitumen extraction. In the embodiment shown in FIG. 2B, thesecond stream 111 can be routed to an upstream process associated withextraction of bitumen from oil sands ore and be mixed with process water126. In a conventional extraction process for oil sands operations, theprocess water 126 can be supplemented/treated with sodium particles(Na⁺) to aid in releasing bitumen from the oil sands ore. Accordingly,one advantage of recycling the second stream 111 to treat the processwater 126 is the ability to decrease any supplement addition of sodiumparticles. Additionally, since the second stream 111 is at leastslightly alkaline due to the excess hydroxide ions present therein,recycling the second stream 111 to the extraction process can increasethe pH of the oil sand ore and thereby improve bitumen extractionefficiency. Yet another advantage of recycling the second stream 111 isthat heat is already present in the second stream 111, and thusrecycling requires less downstream heating requirements compared tousing just the process water 126. Yet another advantage of recycling thesecond stream 111 is removing the volume of the second stream 111 fromthe first stream 109 (i.e., the mixture having the second composition)that is sent downstream to the mixer 112 and the dewatering device 116.Removing the second stream 111 maximizes the solids content of the firststream 109 and minimizes the overall volume of material that is sent tothe dewatering device 116. This decrease in volume can increase overallthroughput of the dewatering system 100, and decrease time and costsassociated with operating the dewatering device 116.

The lime-tailings mixture 109 having the second composition issubsequently directed from the vessel 108 to the mixer 112 where it iscombined with the second dosage of lime 115. The lime slurry 115 used asthe second dosage can include features generally similar or identical tothe lime slurry 105 previously described and used as the first dosage.The mixer 112 and processing conditions (e.g., residence time) of themixture 117 in the mixer 112 can include features generally similar oridentical to the mixer 106 and processing conditions previouslydescribed. The second dosage of lime slurry 115 is added to thelime-tailings mixture to increase the pH of the mixture 117 exiting themixer 112 to be above about 12.0. At this pH, pozzolanic reactions canbegin to occur and thereby chemically modify clay particles from thetailings of the mixture 117.

An advantage of the addition of a first dosage of lime, a polymer, and asecond dosage of lime, as opposed to only a single dosage of lime (e.g.,lime slurry 105), is the decreased cycle time of the overall dewateringprocess. For example, the combination of the lime-tailings mixture 107and the flocculant 110 in the vessel 108 without the significantpresence of soluble calcium ions can result in a quicker settling ofsolids of the lime-tailing mixture 107 in the vessel 108. Additionally,since the second lime dosage 115 is combined with the mixture 109 afterremoving bicarbonate (e.g., via the second stream 111), the bicarbonatedoes not limit the effectiveness of the second lime dosage to promotepozzolanic reactions, as may be the case if only a single lime dosagewas used.

The mixture 117 is subsequently directed (e.g., via gravity and/or apump, from the mixer 112 to the dewatering device 116 or other treatmentprocesses, e.g., via a dewatering device bypass 119. These othertreatment processes can include, for example, thin/thick liftdeposition, deep deposition, or water-capping technologies. Aspreviously mentioned, the dewatering device 116 can include acentrifuge, a filtration system and/or other similar systems that canprovide a physical force on the mixture 117 to promote dewatering andseparate the mixture 117 into a centrate or a filtrate (e.g., therelease water 120) and a cake 118. The centrifuge can include a scrollcentrifugation unit, a solid bowl decanter centrifuge, screen bowlcentrifuge, conical solid bowl centrifuge, cylindrical solid bowlcentrifuge, a conical-cylindrical solid bowl centrifuge, or othercentrifuges used or known in the relevant art. The filtration system caninclude a vacuum filtration system, a pressure filtration system, beltfilter press, or other type of filtering apparatus known in the relevantart that utilizes a desired filtration process. In some embodiments, thefiltration system can include a Whatman 50, 2.7 micron filter and cansubject the lime-tailings mixture to about 100 prig of air pressure.

The mixture 117 may be transferred to the centrifuge or filterimmediately after the mixing process has completed in the mixer 112, orafter a period of time (e.g., a predetermined period of time). In someembodiments, the mixture 117 may, for example, be retained in the mixer112 for one hour, 30 minutes, five minutes, or less. In otherembodiments, the lime-tailings mixture may be retained for more than onehour (e.g., one day, one week, one month, etc.). In general, the mixture117 may be retained for any desired amount of time to ensure it has beenmodified enough for the centrifuge and/or filter to separate asufficient amount of water from the solids in the mixture 117. In someembodiments, the mixture 117 can bypass the dewatering device 116 viastream 119 and instead be directed toward, for example, a tailings pondor settling area to allow the mixture 117 to dewater over time withoutthe use of additional machinery.

The dewatering device 116 has a first outlet used to transfer theseparated release water 120, and a second outlet that is used totransfer the separated cake 118. The separated cake 118 is a solid(e.g., a soft solid) that is composed of the particulate matter found inthe tailings, such as sand, silt, clay, and residual bitumen. The limeadditive particles and some residual water typically do not get removedduring the dewatering process. As previously mentioned, the cake 118 caninclude at least 45% solids by weight. In other embodiments, the cake118 can include at least about 60% solids, at least about 65% solids, atleast about 70% solids, at least about 80% solids, at least about 85%solids, or at least about 90% solids. More generally, the cake 118 mayinclude a greater percentage of solids by weight than the percentage ofliquids by weight.

The separated release water 120 can include water found in the tailings102, water used to dilute the tailings 102 prior to the thickener 108,water added with the flocculants, and/or water that may be found in thelime slurry 104. The separated release water 120 may also contain somesolid particulate matter (e.g., sand, silt, clay, residual bitumen, andlime additive) that is not separated from the release water 120 duringthe dewatering process. In some embodiments, the release water 120includes less than about 10% solids by weight. In other embodiments, therelease water can include less than about 5% solids, less than about 4%solids, less than about 3% solids, or less than 1% solids. In general,the release water 120 includes a significantly greater percentage ofwater by weight than the percentage of solids by weight.

The release water 120 may be directed to a number of differentapplications. For example, the release water 120 may be (a) recycledback to the tailings treatment process, or (b) used to regeneratecaustic soda (e.g., sodium hydroxide) in water utilized in the bitumenextraction process. The release water 120 can be treated with carbondioxide to reduce the pH and amount of soluble calcium cations presenttherein. This can be done via natural absorption of bicarbonates (e.g.,by carbon dioxide present in the atmosphere), or by actively injectingcarbon dioxide into the release water 120. In some embodiments whereinthe release water 120 is recycled back to the tailings treatment processas the recycle stream 122, at least a portion of the release water 120is recycled and added into the tailings holding reservoir 102 or thetailings stream 103 when being transferred to the mixer 106. Therecycled release water 122 mixes with the tailings 103 prior to or whilebeing combined with the first dosage of lime slurry 105. Adding therecycle stream 122 to the tailings stream 103 prior to the mixer 106increases the pH level of the tailings 103 because the recycle stream122 includes soluble calcium cations that were not removed during thedewatering process, and is thus alkaline. As will be discussed ingreater detail below, the calcium ions in the recycle stream 122 readilyreact with bicarbonates present in the tailings stream 103 to forminsoluble compounds that precipitate out of solution and can separatefrom the suspended tailings. Using the recycle water 122 to reduce theamount of bicarbonates in the tailings 103 reduces the amount of thelime slurry 105 needed for enhanced dewatering to occur, which in turncan reduce the cost of the overall dewatering process. In someembodiments, using recycle water 120 to increase the pH level of thetailings 103 can be omitted and the tailings dewatering system 100 maynot use any portion of the release water 120 during the dewateringprocess.

The system 200 can include the control system 130, as previouslydescribed. The control system 130 can be used to control operation ofthe system 200. For example, the control system 130 can control (e.g.,regulate, limit and/or prevent) the flow of fluids (e.g., process 101,tailings stream 103, lime slurry 105, lime-tailings mixtures107/109/117, second stream 111, cake 118, dewatering device bypass 119,release water 120, recycle stream 122, etc.) to and/or from differentunits (e.g., tailings reservoir 102, lime holding tank 104, mixers106/112, vessel 108, dewatering device 116, etc.) of the system 200.Additionally, the control system 130 can control operation of individualunits, such as the mixers 106/112 (e.g., controlling mixing speeds),and/or the dewatering device 116.

FIG. 3 is a block diagram of a method 300 of dewatering a tailingsstream, configured in accordance with an embodiment of the presenttechnology. Process portion 302 includes providing a tailings streamincluding 3-40% solids by weight to a dewatering system (e.g., thesystems 100 or 200). The tailings stream can have a composition similaror identical to the tailings stream 103 previously described. Thetailings stream may operate as a steady state system having a constantfeed or as a batch stream in which tailings are provided to the systemat regular intervals.

Process portion 304 includes combining the tailings stream with a dosageof lime, such as quicklime, limestone, hydrated lime, enhanced hydratedlime, or dolomitic lime, to the tailings stream to form a first mixture.Adding the dosage of lime to the tailings stream increases the pH of thetailings stream to or slightly below 12.0 such that bicarbonates presentin the tailings stream begin to react with and be consumed by calciumcations from the dosage of lime. Notably, at a pH below 12.0, cationexchange can occur, but pozzolanic reactions do not readily occurbecause the amount of soluble Ca²⁺ cations in the lime-tailings mixtureand available to react with clay materials (e.g., Kaolinite(Al₂Si₂O₅(OH)₄)) typically found in tailings streams is limited to lessthan 30 mg/L (e.g., about 25 mg/L, 20 mg/L, 15 mg/L, 10 mg/L). Calciumcations from lime additives are consumed by reactions with bicarbonatesat lower pH. This is different than other calcium cations found ingypsum and calcium chloride that have partially soluble calcium cationsat lower pH. For example, when sodium bicarbonate is exposed to calciumhydroxide, calcium cations bond with carbonate ions and sodiumbicarbonate is converted to sodium carbonate (Na₂CO₃), as seen inReaction 1:

Ca(OH)₂+2NaHCO₃→CaCO₃+Na₂CO₃+2H₂O  (Reaction 1)

The calcium hydroxide will also readily react with the sodium carbonateformed during Reaction 1 to form additional calcium carbonate and sodiumhydroxide (NaOH), as seen in Reaction 2:

Ca(OH)₂+2Na₂CO₃→CaCO₃+2NaOH  (Reaction 2)

The calcium carbonate formed during Reactions 1 and 2 will precipitateout of solution into solid particulate matter. Potassium and calciumbicarbonate will undergo similar reactions with calcium hydroxide. Inaddition to the bicarbonates found in the tailings, atmospheric carbondioxide (CO₂) will dissolve in water that has an alkaline pH level toform carbonic acid (H₂CO₃), which reacts with calcium hydroxide to formcalcium carbonate and water, as shown in Reactions 3 and 4:

CO₂+H₂O→H₂CO₃  (Reaction 3)

Ca(OH)₂+H₂CO₃→CaCO₃+2H₂O  (Reaction 4)

While Reactions 3 and 4 reduce the amount of soluble calcium cationsavailable for cation exchange and pozzolanic reactions to occur, theconcentration of carbon dioxide in the atmosphere is relatively low andlimited by diffusion from the atmosphere into water. As such, Reactions3 and 4 require longer periods of time to have an effect on theconcentration of free calcium cations in the lime-tailings mixture underatmospheric conditions. Reactions 1 and 2, on the other hand, arelimited only by the availability of carbonate ions in the lime-tailingsmixture and occur significantly more readily than cation exchange orpozzolanic reactions, which means that there are very few free calciumcations available to react with clays in the tailings until thecarbonate ions are largely depleted. However, as the amount of limeadditive added to the lime-tailings mixture increases, the pH level ofthe mixture will eventually approach about 12.0, or more particularlyabout 11.8, and the concentration of carbonate ions in the mixture willapproach zero. At this point, the number of free and soluble calciumcations in the water will increase.

In process portion 306, the first mixture can optionally be combinedwith a flocculant. The combination of the first mixture with theflocculant can separate into a first stream (e.g., first stream 109)comprising a second mixture, and a second stream (e.g., second stream111) significantly comprising water having sodium hydroxide particles.The sodium hydroxide particles in the second stream are produced in partfrom Reaction 2 and can be removed from the dewatering process such thatonly the second mixture continues toward the dewatering device.

In process portion 308, a second dosage of lime can be combined with thefirst mixture or the second mixture to produce a third mixture having apH greater than about 12.0 (process portion 408). Specifically, thecalcium hydroxide ions provided via the second lime dosage increase thepH of the third mixture and provide divalent cations that can modify andaffect the stability of fine clay soils in the tailings. As the pHincreases above 11.5, the calcium cations from lime are more soluble dueto the depletion of bicarbonates in process water and can replacecations such as sodium and potassium on the surface of clay soils. As pHlevels increase above 12.0, a chemical modification of the clay'ssurface occurs by pozzolanic reactions. In pozzolanic reactions, solublecalcium cations from the lime react with silicic acid (Si(OH)₄) andaluminate (Al(OH)₄ ⁻) functional groups from the clay materials to formcalcium silicate hydrate (CaH₂SiO₄.2H₂O) and various aluminum hydrates,such as calcium aluminate hydrate. After being chemically modified, thefine clay particles grow in size, decrease their water layer, and can beseparated from the water using a centrifuge or filter, as previouslydescribed. In some embodiments, the pozzolanic reactions may occur afterthe third mixture is centrifuged and/or filtered.

As the pH level of the mixture increases above 11.0, settling of thesolid particulate matter in the second mixture also increases. However,the dewatering rate of the second mixture is still limited at that pH.Once the pH level of the mixture reaches a pH level greater than 12.0(e.g., about 12.3 in some cases), pozzolanic reactions between thedissolved calcium cations and the clay particulate matter begin tooccur. As such, the second dosage of lime is used to increase the pHlevel of the mixture above 12.0.

In systems where the tailings stream is provided as a continuous flow ofoil sands tailings, the lime additive may be a continuous flow of limeadditive that is continuously added and mixed into the tailings stream.In systems where the tailings stream is provided as batches, the limeadditive may be added and mixed into the tailings streams in individualbatches.

After the third mixture has been thoroughly mixed, e.g., in the mixer112, the method proceeds to process portion 310, where the third mixtureis dewatered by separating at least a portion of the solid material fromthe liquid components in the third mixture. As previously described, thedewatering process can comprise a centrifuge and/or filter to forciblyseparate the solid material in the third mixture from the liquidcomponents. Specifically, the centrifuge and/or filtration systemprovide a driving force that promotes dewatering the clay particles viacation exchange and pozzolanic reactions, as previously described. Inother embodiments, the third mixture is dewatered in a dedicateddisposal area by a process, such as thin/thick lift deposition, TRO,AFD, and/or PASS, where atmospheric drying and freeze/thaw treatment(s)to allow the third mixture to dewater over time without the use ofadditional machinery.

After dewatering, the method 300 proceeds to produce a cake with asolids content of at least 40% solids by weight. The solids in the cakeare typically sand, silt, clay, residual bitumen, and the lime additive,along with any other solid particulate matter that is present in eitherthe tailings and/or first and second dosages of lime. The balance of thecake is composed primarily of water that was introduced in either thetailings and/or first and second dosages of lime. As previouslydescribed, the dewatering system also produces a release water streamthat is formed from the tailings water from which the solids areseparated. Converting the solid material found in the oils sandstailings stream into a stream of cake that is at least 55% solids byweight enables significantly easier storage, transport and disposal ofthe solids compared to the solid materials trapped in suspension in theoil sands tailings stream.

FIG. 4 depicts a flow chart 400 for dewatering a tailings stream,configured in accordance with an embodiment of the present technology.As shown in the illustrated embodiment, the flow chart 400 includesproviding a tailings stream (block 402), and adding lime to the tailingsstream to produce a lime-tailings mixture having a first composition(block 404). The pH of the lime-tailings mixture having the firstcomposition is then measured (block 406). Depending on whether themeasured pH is less than a first predetermined threshold (block 408),the system (e.g., the control system 130) may decrease the amount oflime being added to the tailings stream in block 404 (block 410). Insome embodiments, the first predetermined threshold may be a pH lessthan or equal to about 12.0, 11.8. or 11.5. As previously described, apH at or below 12.0, for example, can aid in minimizing the bicarbonatespresent in the tailings 103, which can affect the rate of dewatering inthe downstream process. If the measured pH is less than the firstpredetermined threshold, then the system may proceed without adjustingthe amount of lime being added to the tailings stream in block 404.

The flow chart 400 further includes an optional step of combining thelime-tailings mixture with a polymer. As previously described, in someembodiments, the polymer can promote thickening of the lime-tailingsmixture and allow solids from the lime-tailings mixture to settle fastercompared to if the lime-tailings mixture was treated only with lime oronly with a polymer.

As shown in the illustrated embodiment, additional lime is added toproduce a lime-tailings mixture having a second composition differentthan the first composition (block 414), and the pH of the lime-tailingsmixture having the second composition is then measured (block 416).Depending on whether the measured pH is greater than a secondpredetermined threshold (block 418), the system may increase the amountof lime being added to the lime-tailings mixture in block 414 (block420). In some embodiments, the second predetermined threshold may be apH greater than or equal to about 12.0, 12.2 or 12.4. As previouslydescribed, a pH at or above 12.0, for example, can promote pozzolanicreactions, which can chemically modify clay particles of thelime-tailings mixture and thereby stimulate dewatering of thelime-tailings mixture. If the measured pH is greater than the secondpredetermined threshold, then the system may proceed without adjustingthe amount of lime being added to the lime-tailings mixture in block414. After determining whether the pH is greater than the secondpredetermined threshold, the lime-tailings mixture having the secondcomposition can be dewatered to produce a centrate (e.g., viacentrifugation) or a filtrate (e.g., via pressure filtration), and acake (block 422). Each of the centrate, filtrate and cake can undergofurther processing, as previously described with reference to FIG. 2A.

Example 1—Treatment of FFT with and without Lime Coagulation

FIGS. 5A-5C are images of experimental results related to treatment oftailings stream samples with and without lime over a period of time, inaccordance with embodiments of the present technology. Morespecifically, a comparison test was run to examine the differencebetween treating an FFT sample with and without lime coagulation. Eachof the two FFT samples was diluted with process water to approximately3% solids by weight. The sample on the left side of FIGS. 5A, 5B and 5Cwas coagulated with 1000 mg/kg hydrated lime and flocculated with SNFA3331 polymer at a dose of 250 g/dry tonne FFT solids. The sample on theright side of FIGS. 5A, 5B and 5C was treated only with SNF A3331polymer at a dose of 250 g/tonne FFT solids. Both sample conditions weresimultaneously mixed multiple times by lowering a glass rod with arubber stopper on the bottom through the mixture.

FIG. 5A shows the two samples during the final mixing, FIG. 5B shows thetwo samples a period of time (less than one minute) after the image ofFIG. 5A was taken, and FIG. 5C shows the two samples a period of time(less than one minute) after the image of FIG. 5B was taken. As shown inFIGS. 5B and 5C, the lime treated sample settled into a first (bottom)portion including ultra-fine particles of the FFT, and a second (top)portion including release water of the FFT. Notably, the sample treatedonly with polymer does not exhibit the same settling rate, as FIG. 5Cshows the FFT only slightly settled. Example 1 illustrates in part thatlime coagulation with polymer flocculation, as opposed to just polymerflocculation, can improve fines capture of FFT samples.

Example 2—Treatment of FFT with Lime Coagulation Prior to FlocculantAddition

An experimental study was performed to examine the impact on settlingrate of an FFT sample treated with lime prior to being treated with aflocculant. The FFT samples were diluted to 3% solids by weight, andvarying concentrations of lime were added as a well homogenized slurryto the FFT samples. As shown in Table 1 below, the lime concentrationsincluded 0 mg/kg, 750 mg/kg, 900 mg/kg, 1000 mg/kg and 1250 mg/kg. Thelime slurry comprised a hydrated lime concentration of 5% solids by masswith distilled water. After the slurry was added to the FFT sample, thelime was mixed into the 3% solids FFT sample using a submerged plunger.Subsequently, a 0.5 g/L solution of A3331 polymer was added to themixture to attain 250 g/dry ton solids polymer dosage, and the systemwas again mixed. The total volume of material in the cylinder wasapproximately 1 L. The time (in seconds) needed for the interface(referred to as “the mudline”) between the release water and the settledsolids of the mixture to reach 700 mL (70% of total height) wasrecovered to account for the rate of initial settling, and the mudline(in mL) was recorded after 30 minutes to attain the total capacity ofsettling that had occurred.

FIG. 6 is an image of the experimental results obtained in relation toExample 2. As shown in the FIG. 6, the clarity of the release water isdirectly correlated to the dosage of lime concentration. The improvedclarity of the release water is an indication of reduced turbidity ofthe release water.

Table 1 shows results of Example 2 related to the settling time and themudline. The results in Table 1 indicate that as the concentration oflime addition was increased, (a) the settling time generally decreasedand (b) the amount of settled solids, as indicated by the mudline,generally increased. Accordingly, the experimental results of Example 2indicate that the amount of lime additive directly correlates tosettling rate of the lime-FFT mixture.

TABLE 1 Hydrate Time to settle lime to 70% of Mudline addition totalheight at 30 min mg/kg s mL 0 NA 155 750 8 190 900 5 245 1000 3 245 12504 275

Table 2 shows an analysis of the release water from Example 2. Theresults in Table 2 show that dissolved ion concentration varied fordifferent concentrations of lime concentration addition. For example,calcium concentration of the lime treated FFT mixture initially droppedat the 750 mg/kg lime concentration, but increased at higher limedosages and rose above the initial FFT calcium concentration once the pHexceeded 11.7.

Table 2 also shows an immediate reduction in the magnesium content, asmagnesium concentration decreased from 12 mg/L to 1 mg/L at pHs above10. Without being bound by theory, this is likely because a pH above 10causes the magnesium to precipitate as Mg(OH)₂, and is no longer solublein the release water.

Table 2 also shows that carbonate alkalinity is indirectly correlatedwith lime concentration addition. The initial alkalinity content of 882mg CaCO₃/L at the 0 mg/kg lime dosage decreases drastically to 118CaCO₃/L at the 750 mg/kg lime dosage, and then more gradually to 28CaCO₃/L at the 1250 mg/kg lime dosage. Without being bound by theory,the removal of carbonate alkalinity involves a first reaction whichconverts bicarbonates to carbonates by raising the pH from the hydratedlime addition, and a second reaction which reacts soluble calcium (fromthe initial process water or hydrated lime) with carbonate toprecipitate calcium carbonate. The calcium carbonate then sequesters thecalcium as an insoluble solid.

TABLE 2 Hydrated Lime Dissolved Ion Concentration Carbonate AdditionCa²⁺ Na⁺ K⁺ Mg²⁺ Al³⁺ Cl⁻ SO₄ ²⁻ HCO₃ ⁻ Alkalinity mg/kg mg/L mg/L mg/Lmg/L mg/L mg/L mg/L mg/L mg CaCO₃/L pH 0 31 263 7 12 1 90 99 498 882 9.1750 11 232 4 1 1 90 79 6 118 11.4 900 16 261 8 1 2 89 81 1 38 11.6 100031 246 7 1 0 90 97 1 30 11.7 1250 93 279 8 1 1 90 79 1 28 11.8

Generally speaking, the addition of lime in elevated concentrationsprovides benefits to the settling properties of the FFT solids. The limecauses significant decreases in turbidity of the release water, which isan indication of enhanced capture of the clay particles of the FFTduring flocculation. The calcium particles of the lime react with theFFT to neutralize the anionic charges on the surface of the clays, whichin turn coagulates the particles and improves flocculant performance.This is evident by the definitive mud line shown in FIG. 6 and capturedin Table 1 above.

Furthermore, as shown in Table 2, there appears to be an ideal limedosage that provides the quickest initial settling rate of theflocculated FFT. For example, as shown in Table 2, the ideal lime dosagegenerally occurs after the carbonate alkalinity is substantiallyeliminated, but before soluble calcium concentrations rise to 90 mg/L.This ideal lime dosage resulting in increased fines capture isrepresented by the slightly higher mudlines observed in FIG. 6 and shownin Table 1. The slightly higher mudlines likely result because the solidbed in the lime treated FFT contains all of the solids treated in theexperiment, whereas the solid bed from polymer only treatment may notinclude the fine clays suspended in the release water.

Example 3—Impact of Lime on Cake Consolidation Time and Final CakeSolids

Bench scale pressure filtration tests were run to determine the impactof hydrated lime addition on the cake consolidation time and final cakesolids of FFT. Five trials were conducted, including three trialswithout polymer addition (i.e., Trials 1, 2 and 3) and two trials withpolymer addition (i.e., Trials 5 and 6). As seen in Trials 1, 2 and 3,as the dose of hydrated lime increased, the cake consolidation timedecreased and the cake solids content increased. FFT was treated byadding 5% hydrated lime slurry to undiluted FFT with solids contentsmeasured between 32.5 to 35.2%.

Trials 4 and 5 were conducted to examine the impact of increasing thepercent solids of the pressure filter feed. The FFT samples were dilutedto 3% solids to simulate thickener feed. Trial 4 utilized A3331 polymer(175 g/tonne) to flocculate the FFT and increase the underflow solids to40%. Trial 5 utilized approximately 2,000 mg/kg of hydrated lime, whichwas added prior to the A3331 polymer addition. The remaining hydratedlime of 2,000 mg/kg, which was required to achieve a pH over 12, wasadded to the 40% solids thickened underflow.

As shown in Table 3 below, results of Example 3 show that despite higherpercent solids for the feed to the pressure filter in Trial 4,thickening only with polymer increased the cycle time and decreased thepressure filter cake solids level. Furthermore, as shown in Trial 5,adding hydrated lime both before and after the polymer addition resultedin the best cake consolidation time of 40 minutes, and final cake solidscontent of 72.0%.

TABLE 3 No Polymer Thickened Added Tailings Parameter 1 2 3 4 5 FeedSolids 35.2 34.1 32.5 40.0 40.0 Concentration (wt %) Lime Dose (mg/kg)2000 4000 7000 0 2000/2000 Chamber Thickness 25.0 25.0 25.0 25.0 25.0(mm) Cake Consolidation Time 65.0 59.0 50.0 85.0 40.0 (min) Final CakeSolids (%) 63.6 71.2 73.5 65.8 72.0

Throughout this disclosure, the singular terms “a,” “an,” and “the”include plural referents unless the context clearly indicates otherwise.Similarly, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twoor more items, then the use of “or” in such a list is to be interpretedas including (a) any single item in the list, (b) all of the items inthe list, or (c) any combination of the items in the list. The term“about,” as used throughout this application, is meant to indicate arange of +/−10% of the indicated value. Additionally, the terms“comprising” and the like are used throughout this disclosure to meanincluding at least the recited feature(s) such that any greater numberof the same feature(s) and/or one or more additional types of featuresare not precluded. Reference herein to “one embodiment,” “anembodiment,” or similar formulations means that a particular feature,structure, operation, or characteristic described in connection with theembodiment can be included in at least one embodiment of the presenttechnology. Thus, the appearances of such phrases or formulations hereinare not necessarily all referring to the same embodiment. Furthermore,various particular features, structures, operations, or characteristicsmay be combined in any suitable manner in one or more embodiments.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Additionally, aspects of the invention describedin the context of particular embodiments or examples may be combined oreliminated in other embodiments. Although advantages associated withcertain embodiments of the invention have been described in the contextof those embodiments, other embodiments may also exhibit suchadvantages. Additionally, not all embodiments need necessarily exhibitsuch advantages to fall within the scope of the invention. Accordingly,the invention is not limited except as by the appended claims.

EXAMPLES OF THE PRESENT TECHNOLOGY

The subject technology is illustrated, for example, according to variousexamples described below. Various examples of the subject technology aredescribed as numbered clauses (1, 2, 3, etc.) for convenience. These areprovided as examples and do not limit the subject technology. It isnoted that any of the dependent clauses may be combined in anycombination, and placed into a respective independent clause (e.g.,clause 1, 22, 25, etc.). The other clauses can be presented in a similarmanner.

1. A method for treating a tailings stream, the method comprising:

-   -   adding a first dosage of lime additive to a tailings stream to        produce a first mixture having a pH less than about 12.0 and a        soluble calcium level less than about 100 mg/L;    -   after adding the first dosage, adding a second dosage of lime        additive to the first mixture to produce a second mixture having        a pH greater than about 12.0; and    -   dewatering the second mixture to produce a cake including at        least 40% solids by total weight.

2. The clause of claim 1, further comprising:

-   -   directing the first mixture to a thickener vessel; and    -   separating the first mixture into a first stream comprising        water and a second stream,    -   wherein adding the second dosage to the first mixture includes        adding the second dosage to the second stream.

3. The clause of claim 2, further comprising directing the first streamto be mixed with the first dosage prior to adding the first dosage tothe tailings stream.

4. The clause of claim 2 wherein the first stream has a soluble calciumlevel within a range from about 10 mg/L to about 30 mg/L.

5. The clause of claim 1, further comprising adding a flocculant slurrycontaining one or more polymers to the first mixture prior to adding thesecond dosage, wherein the first mixture having the added flocculantslurry includes a solids content exceeding 30% by wet weight.

6. The clause of claim 5 wherein the first dosage and the tailings arecombined in a first mixer, the flocculant slurry is added to the firstmixture in or before a thickener vessel, and the second dosage is addedto the second mixture in a second mixer downstream of the thickenervessel.

7. The clause of claim 1 wherein the cake comprises a first stream, andwherein dewatering includes filtering or centrifuging the second mixtureto produce the first stream and a second stream, the second streamcomprising release water having soluble calcium ions.

8. The clause of claim 7 wherein the tailings stream includesbicarbonates, the method further comprising:

recycling at least a portion of the second stream to be mixed with thetailings stream prior to the addition of the first dosage, whereinrecycling the second stream increases the pH of the tailings stream andreduces the bicarbonates of the tailings stream.

9. The clause of claim 8, further comprising:

-   -   after recycling the second stream, decreasing the amount of the        first or second dosages as a result of the second stream        increasing the pH of the tailings stream.

10. The clause of claim 8 wherein recycling the second stream is basedat least in part on a measured soluble calcium level of the secondstream.

11. The clause of claim 8 wherein the recycled second stream is added tothe first dosage prior to the first dosage being added to the tailings.

12. The clause of claim 1 wherein at least one of the first or seconddosages are part of a lime slurry including hydrated lime, wherein thehydrated lime includes particles having an average surface area greaterthan or equal to about 30 m²/g.

13. The clause of claim 1 wherein at least one of the first or seconddosages includes a lime slurry comprising less than 15% lime by totalweight.

14. The clause of claim 1, further comprising:

-   -   forming a flocculant slurry by combining one or more polymers        with at least one of process water or makeup water; and    -   adding the flocculant slurry to the first mixture prior to        adding the second dosage.

15. The clause of claim 1 wherein the first mixture has a pH less thanabout 11.5 and the second mixture has a pH greater than about 12.4.

16. The clause of claim 1 wherein the amount of the second dosage addedis based at least in part on a pH of the second mixture.

17. The clause of claim 1 wherein the cake comprises a thickenedtailings stream, the method further comprising:

-   -   directing the thickened tailings stream to at least one of a        deposition process or a water-capped cell.

18. The clause of claim 1 wherein the second mixture includes silicicacid or aluminate, and wherein dewatering the second mixture includesreacting the lime of the second dosage with the silicic acid oraluminate.

19. The clause of claim 1 wherein the first mixture includes a solublecalcium level less than 50 mg/L.

20. The clause of claim 1 wherein the tailings stream includesbicarbonates, and wherein adding the first dosage of lime comprisesreducing the bicarbonates in the tailings stream to be below about 20mg/L.

21. The clause of claim 1 wherein the first mixture includes alkalinity,and wherein adding the first dosage comprises reducing the alkalinity ofthe first mixture to be below about 130 mg/L of calcium carbonateequivalent.

22. A system for treating tailings streams for oil sands or miningoperations, the system comprising:

-   -   a tailings reservoir including tailings having about 3-40%        solids by total weight;    -   a first mixer positioned to receive a first lime slurry and the        tailings from the tailings reservoir;    -   a second mixer downstream of the first mixer and in fluid        communication with the first mixer;    -   a dewatering device downstream of and in fluid communication        with the second mixer, the dewatering device comprising at least        one of a centrifuge or filter; and    -   a computer-readable medium having instructions that, when        executed, cause the system to—        -   add the first lime slurry to the tailings in or before the            first mixer to produce a first mixture in the first mixer,            wherein adding the first lime slurry is based at least in            part on a pH of the first mixture being less than about            12.0;        -   add the second lime slurry to the tailings in or before the            second mixer to produce a second mixture in the second            mixer, wherein adding the second lime slurry is based at            least in part of a pH of the second mixture being greater            than about 12.0; and        -   operate the dewatering device to produce a centrate or            filtrate comprising water and a cake comprising at least 40%            solids by total weight.

23. The clause of claim 22, further comprising a thickener vesselbetween the first and second mixers, the instructions, when executed,further cause the system to add a flocculant slurry comprising apolymer.

24. The clause of claim 22, further comprising a recycle stream in fluidcommunication with the centrate or filtrate, wherein the instructions,when executed, further cause the system to direct, via the recyclestream, at least a portion of the centrate or filtrate to be mixed withthe tailings, the first lime slurry and/or the second lime slurry.

25. A method for treating tailings streams, the method comprising:

-   -   adding a first dosage of lime additive to a tailings stream to        produce a first mixture, the first mixture having a pH less than        about 12.0 and a soluble calcium level less than 100 mg/L;    -   combining the first mixture with a flocculant slurry comprising        one or more polymers;    -   separating the combined first mixture and flocculant slurry into        a first stream and a second stream; and adding a second dosage        of lime additive to the second stream to produce a second        mixture having a pH greater than about 12.0.

26. The clause of claim 25, further comprising dewatering the secondmixture to produce a centrate or filtrate comprising water and a cakeincluding at least 40% solids by total weight, the method furthercomprising recycling at least a portion of the centrate or filtrate tobe mixed with at least one of the first dosage, the tailings stream orthe second dosage.

27. The clause of claim 25 wherein the first mixture has a pH less than11.5 and the second mixture has a pH greater than 12.4.

28. The clause of claim 25, further comprising measuring a pH of thefirst mixture, wherein adding the first dosage to the tailings stream isbased at least in part on the measured pH of the first mixture.

29. The clause of claim 28, further comprising measuring a pH of thesecond mixture, wherein adding the second dosage to the second mixtureis based at least in part on the measured pH of the second mixture.

30. The clause of claim 25 wherein the tailings includes a firstconductivity and the first stream includes a second conductivity lessthan the first conductivity.

31. The clause of claim 25 wherein the tailings includes a first totaldissolved solids content and the first mixture includes a second totaldissolved solids content less than the first total dissolved solidscontent.

32. The clause of claim 25 wherein the first mixture includes amagnesium content less than 20 mg/L.

33. The clause of claim 25 wherein the first stream comprises water andthe second stream comprises thickened tailings, the method furthercomprising:

-   -   adding process water to the tailings stream; and    -   recycling at least a portion of the first stream to be mixed        with the process water prior to adding the process water to the        tailings stream.

Additional features and advantages of the subject technology aredescribed below, and in part will be apparent from the description, ormay be learned by practice of the subject technology. The advantages ofthe subject technology will be realized and attained by the structureparticularly pointed out in the written description and claims hereof aswell as the appended drawings.

I/We claim:
 1. A method for treating a tailings stream, the methodcomprising: adding a first dosage of lime additive to a tailings streamto produce a first mixture having a pH less than about 12.0 and asoluble calcium level less than about 100 mg/L; after adding the firstdosage, adding a second dosage of lime additive to the first mixture toproduce a second mixture having a pH greater than about 12.0; anddewatering the second mixture to produce a cake including at least 40%solids by total weight.
 2. The method of claim 1, further comprising:directing the first mixture to a thickener vessel; and separating thefirst mixture into a first stream comprising water and a second stream,wherein adding the second dosage to the first mixture includes addingthe second dosage to the second stream.
 3. The method of claim 2,further comprising directing the first stream to be mixed with the firstdosage prior to adding the first dosage to the tailings stream.
 4. Themethod of claim 2 wherein the first stream has a soluble calcium levelwithin a range from about 10 mg/L to about 30 mg/L.
 5. The method ofclaim 1, further comprising adding a flocculant slurry containing one ormore polymers to the first mixture prior to adding the second dosage,wherein the first mixture having the added flocculant slurry includes asolids content exceeding 30% by wet weight.
 6. The method of claim 5wherein the first dosage and the tailings are combined in a first mixer,the flocculant slurry is added to the first mixture in or before athickener vessel, and the second dosage is added to the second mixturein a second mixer downstream of the thickener vessel.
 7. The method ofclaim 1 wherein the cake comprises a first stream, and whereindewatering includes filtering or centrifuging the second mixture toproduce the first stream and a second stream, the second streamcomprising release water having soluble calcium ions.
 8. The method ofclaim 7 wherein the tailings stream includes bicarbonates, the methodfurther comprising: recycling at least a portion of the second stream tobe mixed with the tailings stream prior to the addition of the firstdosage, wherein recycling the second stream increases the pH of thetailings stream and reduces the bicarbonates of the tailings stream. 9.The method of claim 8, further comprising: after recycling the secondstream, decreasing the amount of the first or second dosages as a resultof the second stream increasing the pH of the tailings stream.
 10. Themethod of claim 8 wherein recycling the second stream is based at leastin part on a measured soluble calcium level of the second stream. 11.The method of claim 8 wherein the recycled second stream is added to thefirst dosage prior to the first dosage being added to the tailings. 12.The method of claim 1 wherein at least one of the first or seconddosages are part of a lime slurry including hydrated lime, wherein thehydrated lime includes particles having an average surface area greaterthan or equal to about 30 m²/g.
 13. The method of claim 1 wherein atleast one of the first or second dosages includes a lime slurrycomprising less than 15% lime by total weight.
 14. The method of claim1, further comprising: forming a flocculant slurry by combining one ormore polymers with at least one of process water or makeup water; andadding the flocculant slurry to the first mixture prior to adding thesecond dosage.
 15. The method of claim 1 wherein the first mixture has apH less than about 11.5 and the second mixture has a pH greater thanabout 12.4.
 16. The method of claim 1 wherein the amount of the seconddosage added is based at least in part on a pH of the second mixture.17. The method of claim 1 wherein the cake comprises a thickenedtailings stream, the method further comprising: directing the thickenedtailings stream to at least one of a deposition process or awater-capped cell.
 18. The method of claim 1 wherein the second mixtureincludes silicic acid or aluminate, and wherein dewatering the secondmixture includes reacting the lime of the second dosage with the silicicacid or aluminate.
 19. The method of claim 1 wherein the first mixtureincludes a soluble calcium level less than 50 mg/L.
 20. The method ofclaim 1 wherein the tailings stream includes bicarbonates, and whereinadding the first dosage of lime comprises reducing the bicarbonates inthe tailings stream to be below about 20 mg/L.
 21. The method of claim 1wherein the first mixture includes alkalinity, and wherein adding thefirst dosage comprises reducing the alkalinity of the first mixture tobe below about 130 mg/L of calcium carbonate equivalent.
 22. A systemfor treating tailings streams for oil sands or mining operations, thesystem comprising: a tailings reservoir including tailings having about3-40% solids by total weight; a first mixer positioned to receive afirst lime slurry and the tailings from the tailings reservoir; a secondmixer downstream of the first mixer and in fluid communication with thefirst mixer; a dewatering device downstream of and in fluidcommunication with the second mixer, the dewatering device comprising atleast one of a centrifuge or filter; and a computer-readable mediumhaving instructions that, when executed, cause the system to— add thefirst lime slurry to the tailings in or before the first mixer toproduce a first mixture in the first mixer, wherein adding the firstlime slurry is based at least in part on a pH of the first mixture beingless than about 12.0; add the second lime slurry to the tailings in orbefore the second mixer to produce a second mixture in the second mixer,wherein adding the second lime slurry is based at least in part of a pHof the second mixture being greater than about 12.0; and operate thedewatering device to produce a centrate or filtrate comprising water anda cake comprising at least 40% solids by total weight.
 23. The system ofclaim 22, further comprising a thickener vessel between the first andsecond mixers, the instructions, when executed, further cause the systemto add a flocculant slurry comprising a polymer.
 24. The system of claim22, further comprising a recycle stream in fluid communication with thecentrate or filtrate, wherein the instructions, when executed, furthercause the system to direct, via the recycle stream, at least a portionof the centrate or filtrate to be mixed with the tailings, the firstlime slurry and/or the second lime slurry.
 25. A method for treatingtailings streams, the method comprising: adding a first dosage of limeadditive to a tailings stream to produce a first mixture, the firstmixture having a pH less than about 12.0 and a soluble calcium levelless than 100 mg/L, combining the first mixture with a flocculant slurrycomprising one or more polymers; separating the combined first mixtureand flocculant slurry into a first stream and a second stream; andadding a second dosage of lime additive to the second stream to producea second mixture having a pH greater than about 12.0.
 26. The method ofclaim 25, further comprising dewatering the second mixture to produce acentrate or filtrate comprising water and a cake including at least 40%solids by total weight, the method further comprising recycling at leasta portion of the centrate or filtrate to be mixed with at least one ofthe first dosage, the tailings stream or the second dosage.
 27. Themethod of claim 25 wherein the first mixture has a pH less than 11.5 andthe second mixture has a pH greater than 12.4.
 28. The method of claim25, further comprising: measuring a pH of the first mixture, whereinadding the first dosage to the tailings stream is based at least in parton the measured pH of the first mixture; and measuring a pH of thesecond mixture, wherein adding the second dosage to the second mixtureis based at least in part on the measured pH of the second mixture. 29.The method of claim 25 wherein: the tailings includes a firstconductivity and the first stream includes a second conductivity lessthan the first conductivity, the tailings includes a first totaldissolved solids content and the first mixture includes a second totaldissolved solids content less than the first total dissolved solidscontent, and the first mixture includes a magnesium content less than 20mg/L.
 30. The method of claim 25 wherein the first stream compriseswater and the second stream comprises thickened tailings, the methodfurther comprising: adding process water to the tailings stream; andrecycling at least a portion of the first stream to be mixed with theprocess water prior to adding the process water to the tailings stream.